There is an urgent need to identify new and effective drugs for small cell lung cancer (SCLC), a tumor characterized by aggressive growth, early metastases, and a 5-year survival rate of less than 2%. While it is initially chemosensitive, with 70% response rates to first-line therapy of cisplatin plus etoposide, most patients relapse after initial therapy with drug-resistant tumors. Dozens of new drugs have been tested for activity in SCLC over the decades, including more than 40 agents that have failed in phase III trials. None of the targeted drugs used in non-small cell lung cancer or other solid tumors are effective in SCLC. An outstanding feature of SCLC is the near uniform bi-allelic genetic inactivation of RB1 and TP53. However, since genetically inactivated RB1 and TP53 cannot be reactivated nor is it clinically feasible to reintroduce them into SCLC cells, this defining genetic feature has not led to treatment strategies for SCLC. In this grant, we will test the hypothesis that inhibiting a downstream target of Rb1 can re-establish the tumor suppressor and pro-apoptotic actions that were lost when Rb1 was inactivated. In preliminary studies, we found that modulation of only one downstream target of Rb1, the E3 ubiquitin ligase SCFSkp2/Cks1 (Skp2), can dramatically block the pro-tumorigenic consequences of the loss of Rb1, and induce Rb1-Skp2 synthetic lethal apoptosis in SCLC. This is due to the role Rb1 and Skp2 play in the regulation of p27 (CDKN1B), whereby the loss of Rb1 leads to the Skp2-mediated ubiquitination and degradation of p27 and the subsequent loss of cell cycle regulation. Most critically, we have identified active, small molecule Skp2 inhibitors to specifically target this vulnerability. The specific aims are: 1) To determine the role of the Cks1-Skp2 interaction in Rb1 and p53 deletion- induced SCLC. If the role is found to be essential, we will have identified another target to inhibit Skp2-medited p27 ubiquitination. Since some human SCLCs with p53 missense mutations may have gained new oncogenic functions (GOF), in addition to the loss of classic p53 functions, we will determine if these p53 GOF mutations affects the sensitivity to Skp2 inhibition. 2) To determine effects of Skp2 inactivation using genetic and pharmacologic approaches. Four different small molecule Skp2 inhibitors, with distinct molecular targets, will be used in several novel in vitro and in vivo models. Particular focus will be on SCLC liver metastasis, and on comparisons between chemotherapy-nave and chemo-resistant SCLC cells. 3) To determine the antitumor effects of Skp2 inhibitors in a more clinically relevant mouse tumor model using a large panel of SCLC PDXs (patient-derived xenografts). Comparisons will be made between PDXs derived from chemotherapy-nave and chemo-resistant SCLC tumors, including ?isogenic? PDXs derived from serial specimens from the same patient pre- and post-chemotherapy. When completed, research in this proposal will potentially benefit most SCLC patients since bi-allelic inactivation of RB1 and TP53 is nearly uniform in SCLC.